A fluorescence conversion substrate which converts the wavelength of light emitted from a light source part using a fluorescent material has been applied in various fields such as the electronic display field.
For example, an organic electroluminescent device (“electroluminescent” may be hereinafter referred to as “EL”) has been disclosed which has an organic electroluminescent material part which emits light of blue or bluish green and a fluorescent material part which absorbs light emitted from the emitting layer and emits visible fluorescence of at least one color from bluish green to red (see Patent Document 1, for example).
This method utilizes a blue light source and converts the color of light using a color conversion medium to obtain the three primary colors. Specifically, a fluorescent dye in the color conversion medium is excited by irradiation of blue light, whereby green or red light having a longer wavelength is generated.
Organic fluorescent dyes and organic fluorescent pigments have been generally used as the fluorescent materials for the fluorescence conversion medium. For example, a red color conversion medium has been disclosed which is obtained by dispersing a rhodamine fluorescent pigment and a fluorescent pigment having an absorption in the blue region and inducing energy transfer or reabsorption in the rhodamine fluorescent pigment in a light-transmitting medium (see Patent Document 2, for example).
To enhance the fluorescence conversion efficiency as well as to increase the intensity of the converted light (fluorescent intensity), it is required to allow light emitted from a light source to be absorbed sufficiently in the fluorescence conversion medium. However, if the concentration of organic fluorescent dyes in the fluorescence conversion medium is increased, the organic fluorescent dyes may contact each other inside the film. As a result, energy absorbed from the light source is transferred to adjacent dyes (concentration quenching) so that a fluorescent quantum efficiency cannot be obtained.
When a reactive resin such as a photo-curable resin or a heat-curable resin is used as a light-transmitting medium, a reactive component in the resin and an organic fluorescent dye are reacted to decompose or undergo a structural change. Therefore, there was a problem that the fluorescent intensity of a fluorescence conversion medium is lowered when an ultraviolet light is irradiated or when baking is performed at a high temperature, for example, at 200° C., during the process of forming a fluorescence conversion medium. When an emitting apparatus is driven continuously, a fluorescence conversion medium is continuously irradiated with exited light, resulting in lowering of the fluorescent intensity of a fluorescence conversion medium with the lapse of time.
To solve the above-mentioned problems, specifically, to solve the problems caused by the use of an organic fluorescent dye as a fluorescent material used in a fluorescence conversion medium, Patent Document 3 proposes a technique of fabricating a full-color organic electroluminescence (EL) device utilizing an inorganic nanocrystal. According to this technique, a film obtained by dispersing CdS, CdSe, or CdTe as a nanocrystal in a light-transmitting resin is used as a fluorescence conversion medium, and the fluorescence conversion medium is connected to an organic EL device emitting a blue monochromatic color with a peak wavelength of 450 nm, whereby emission of red light and green light is achieved. The colors obtained by conversion such as red and green are controlled by adjusting the particle size of the inorganic nanocrystal.
Patent Document 4 discloses a technique of combining a fluorescence conversion medium in which an inorganic nanocrystal is dispersed with an LED to realize a highly efficient white LED.
In principle, a fluorescent fine particle formed of an inorganic nanocrystal has a large absorption in the emission band. Therefore, even though an attempt is made to increase the concentration of fluorescent fine particles to enhance conversion efficiency, improvement of the conversion efficiency is limited due to self absorption. As a result, the conversion efficiency is not improved to a degree which can be expected from the fluorescent quantum efficiency of a nanocrystal.
Patent Document 5 discloses a color emitting apparatus obtained by combining an organic EL light source and a fluorescence conversion medium obtained by dispersing an inorganic nanocrystal, which realizes high fluorescence conversion efficiency and high durability. However, loss of emitting components due to self absorption cannot be avoided, and therefore, fluorescence conversion efficiency was not sufficient.
[Patent document 1]JP-A-H3-152897[Patent document 2]JP-A-H8-286033[Patent document 3]U.S. Pat. No. 6,608,439[Patent document 4]U.S. Pat. No. 6,501,091[Patent document 5]WO2005/097939
The invention has been made in view of the aforementioned problems. An object of the invention is to provide a fluorescence conversion medium which exhibits high fluorescence conversion efficiency, as well as to provide a color emitting apparatus utilizing the same.